The present invention relates generally to fastener-driving tools used to drive fasteners into workpieces, and specifically to pneumatic or combustion-powered fastener-driving tools, also referred to as fastener drivers.
Combustion-powered tools are known in the art. Exemplary tools are manufactured by Illinois Tool Works, Inc. of Glenview, Ill. for use in driving fasteners into workpieces, and are described in commonly assigned patents to Nikolich U.S. Pat. Re. No. 32,452, and U.S. Pat. Nos. 4,522,162; 4,483,473; 4,483,474; 4,403,722; 5,133,329; 5,197,646; 5,263,439; 6,145,724 and 7,383,974 all of which are incorporated by reference herein.
Such tools incorporate an external tool housing enclosing a small internal combustion engine. The engine is powered by a canister of pressurized fuel gas, also called a fuel cell. A battery-powered electronic power distribution unit produces a spark for ignition, and a fan located in a combustion chamber provides for both an efficient combustion within the chamber, while facilitating processes ancillary to the combustion operation of the device. Such ancillary processes include: cooling the engine, mixing the fuel and air within the chamber, and removing, or scavenging, combustion by-products. The engine includes a reciprocating piston with an elongated, rigid driver blade disposed within a single cylinder body.
A valve sleeve is axially reciprocable about the cylinder and, through a linkage, moves to close the combustion chamber when a work contact element at the end of the linkage is pressed against a workpiece. This pressing action also triggers a fuel-metering valve to introduce a specified volume of fuel into the closed combustion chamber. This same movement of the tool against the workpiece causes the fan inside the combustion chamber to turn on and mix the fuel with the air inside the combustion chamber.
Upon the pulling of a trigger, which closes a trigger switch, a spark is generated for igniting a charge of gas in the combustion chamber of the engine, the resulting high pressure inside the chamber causes the combined piston and driver blade to be forced downward to impact a positioned fastener and drive it into the workpiece. Just before the piston impacts a resilient bumper at a lower end of the cylinder, the piston passes an exhaust port, through which some of the exhaust gas is vented. Next, the tool valve sleeve and cylinder absorb heat from the combustion to generate vacuum pressure that pulls the piston back to its uppermost position for the next cycle. Fasteners are fed magazine-style into the nosepiece, where they are held in a properly positioned orientation for receiving the impact of the driver blade.
For efficient operation, it is preferred that the combustion chamber remains sealed until the piston returns to its uppermost or pre-firing position. The amount of time that the combustion chamber remains closed is a function of the operator's work rhythm and is often too short when attempting a repetitive cycle operation, where the trigger remains pulled and the workpiece contact element (WCE) is rapidly pressed upon the workpiece for fastener driving, and then the tool is quickly lifted and moved to the next fastener location.
With combustion-powered tools of the type disclosed in the patents incorporated by reference above, by firing rate and control of the valve sleeve, the operator controls the time interval provided for the vacuum-type piston return. The formation of the vacuum occurs following the combustion of the mixture and the exhausting of the high-pressure burnt gases. With residual high temperature gases in the tool, the surrounding lower temperature aluminum components cool and collapse the gases, thereby creating a vacuum. In many cases, such as in trim applications, the operator's cycle rate is slow enough that vacuum return works consistently and reliably.
However, for those cases where a tool is operated at a much higher cycle rate, the operator can open the combustion chamber during the piston return cycle by removing the tool from the workpiece. This causes the vacuum to be lost and piston travel will stop before reaching the top of the cylinder. This leaves the driver blade in the guide channel of the nosepiece, thereby preventing the nail strip from advancing towards the nose. The net result is no nail in the firing channel and no nail fired in the next shot.
To assure adequate closed combustion chamber dwell time in the sequentially-operated combustion tools identified above, a chamber lockout device is known that is linked to the trigger. This mechanism holds the combustion chamber closed until the operator releases the trigger. This extends the dwell time (during which the combustion chamber is closed) by taking into account the operator's relatively slow musculature response time. In other words, the physical release of the trigger consumes enough time of the firing cycle to assure piston return. The mechanism also maintains a closed chamber in the event of a large recoil event created, for example, by firing into hard wood or on top of another nail. It is disadvantageous to maintain the chamber closed longer than the minimum time to return the piston, as cooling and purging of the tool is prevented.
In commonly-assigned U.S. Pat. No. 7,383,974, an electromagnetic solenoid controls a pivoting latch for periodically locking the valve sleeve in the closed position. In some cases, electromagnetic force has been found to lack sufficient holding power for retaining the valve sleeve against motion along the main tool axis towards the open position of the valve sleeve.
Thus, there is a need for a combustion-powered fastener-driving tool which is capable of operating in a repetitive cycle mode. There is also a need for a combustion-powered fastener-driving tool which addresses the special needs of delaying the opening of the combustion chamber to achieve complete piston return in a repetitive cycle mode.